BoyntonStu
04-03-2009, 10:11 AM
Copied from From Watercar Forum:
"
With 8 plates, 7 cells, a PWM should not be needed. That is 2 Volts per
cell at 14 Volts, unless you have a bad regulator on the alternator, you
should be just under that (typically 13.8 Volts.) At 7 plates, 6 cells,
you will see 2.333 Volts per cell or 12 Volts at full output for 2 Volts
per cell.
For a 100 Hz PWM with 1 Ampere of ripple, you would need a fairly large
inductor (on the order of 1/10 of a Henry) and it would have to carry
all the current, since you would have a load with a large voltage drop
and a high duty cycle. To see what a choke is, see
http://en.wikipedia.org/wiki/Inductor. The magic formula is V=LdI/dT
where V is the voltage, L is the current. dI is the current change and
dT is the time change. Solving for L we get L=V*dT/dI, let's assume a 1
Ampere current change and a 12 Volt gas generator voltage while running.
A faster switch will require faster switching or the peak losses will
skyrocket. The charge in the gate to drain capacitor results in slower
switching unless the drive is fast and of high current *DURING
SWITCHING*. Calculating it at 500 KHz we get 12*2^-6/1 or 24
microhenries That is about 44 turns of #14 wire on a T300-2 core --
over 3" in diameter! As you can see, high power is costly.
So we also need drivers, a FET, a catch diode, etc. Bet it runs over
$100 before you are done.
Assuming you can find one (without design costs), you are talking a
non-trivial component which will work well.
Now, if you wanted to build a 12 plate cell, you could use a boost
regulator and get higher output for the same price. Boost regulators
take 14 Volts in and give higher voltage (at lower current) out. Given
the same POWER input, the larger plate area means less bubble shrouding
and higher gas output. Not much, but enough to make it somewhat better.
The simple answer is that you don't need a PWM if you design an 8 plate
system. It won't run well unless you have a 14 Volt supply for it (a
car battery with a small charger won't hack it, you will get 12 Volts,
you need an alternator and engine to drive it or a really big (i.e. 50
Ampere) charger.)
Be sure to add a pressure switch to ensure the cell shuts down if the
pressure "runs away" during operation.
And be sure your bubbler has both an anti-siphon check valve and an
vacuum breaker check valve. Smack found without the vacuum breaker
check valve, his cell was damaged. Without an anti-siphon valve, your
bubbler will empty into the cell and without a bubbler working, you are
asking for trouble! I have put a schematic in the files area showing
how check valves help. /* Check valves can not stop a flashback!*/
--
David G. LeVine
Nashua, NH 03060"
--------------------------------------------
The point: Use 7 cells and you will not need a PWM.
Small point: I found that with 1/32" spacing, the Amoeba Cell can go lower than 2 V/cell (1.88V)
BoyntonStu
"
With 8 plates, 7 cells, a PWM should not be needed. That is 2 Volts per
cell at 14 Volts, unless you have a bad regulator on the alternator, you
should be just under that (typically 13.8 Volts.) At 7 plates, 6 cells,
you will see 2.333 Volts per cell or 12 Volts at full output for 2 Volts
per cell.
For a 100 Hz PWM with 1 Ampere of ripple, you would need a fairly large
inductor (on the order of 1/10 of a Henry) and it would have to carry
all the current, since you would have a load with a large voltage drop
and a high duty cycle. To see what a choke is, see
http://en.wikipedia.org/wiki/Inductor. The magic formula is V=LdI/dT
where V is the voltage, L is the current. dI is the current change and
dT is the time change. Solving for L we get L=V*dT/dI, let's assume a 1
Ampere current change and a 12 Volt gas generator voltage while running.
A faster switch will require faster switching or the peak losses will
skyrocket. The charge in the gate to drain capacitor results in slower
switching unless the drive is fast and of high current *DURING
SWITCHING*. Calculating it at 500 KHz we get 12*2^-6/1 or 24
microhenries That is about 44 turns of #14 wire on a T300-2 core --
over 3" in diameter! As you can see, high power is costly.
So we also need drivers, a FET, a catch diode, etc. Bet it runs over
$100 before you are done.
Assuming you can find one (without design costs), you are talking a
non-trivial component which will work well.
Now, if you wanted to build a 12 plate cell, you could use a boost
regulator and get higher output for the same price. Boost regulators
take 14 Volts in and give higher voltage (at lower current) out. Given
the same POWER input, the larger plate area means less bubble shrouding
and higher gas output. Not much, but enough to make it somewhat better.
The simple answer is that you don't need a PWM if you design an 8 plate
system. It won't run well unless you have a 14 Volt supply for it (a
car battery with a small charger won't hack it, you will get 12 Volts,
you need an alternator and engine to drive it or a really big (i.e. 50
Ampere) charger.)
Be sure to add a pressure switch to ensure the cell shuts down if the
pressure "runs away" during operation.
And be sure your bubbler has both an anti-siphon check valve and an
vacuum breaker check valve. Smack found without the vacuum breaker
check valve, his cell was damaged. Without an anti-siphon valve, your
bubbler will empty into the cell and without a bubbler working, you are
asking for trouble! I have put a schematic in the files area showing
how check valves help. /* Check valves can not stop a flashback!*/
--
David G. LeVine
Nashua, NH 03060"
--------------------------------------------
The point: Use 7 cells and you will not need a PWM.
Small point: I found that with 1/32" spacing, the Amoeba Cell can go lower than 2 V/cell (1.88V)
BoyntonStu